1. Field of the Invention
The present invention relates to a method and a system for calibration of semiconductor testing equipment (“tester”) using photoemission detection of switching devices, especially for calibrating signal timing and characteristics in such devices.
2. Description of the Related Art
Testing Equipment (testers) are well known in the art for measuring performance of, and debugging semiconductor circuits. Examples of such equipments are NPTest 9000 available from NPTest of San Jose, Calif., and Vanguard available from IMS (a Credence Company) of Hillsboro, Oreg. FIG. 1 depicts a general schematic of a tester 100, consisting of a controller (may be a tailor-programed general-purpose computer) 110, a test head 120 having a tester load board adapter 125. For debug of an integrated circuit (IC), the IC is attached to a DUT socket (not shown), which is mounted onto the load board 130. The load board 130 is electrically connected to the test adapter 125 via, e.g., cable interface (not shown). The controller 110 then generates various test signals that are transmitted to the IC via the test adapter 125 and load board 130. The tests comprise switching of the IC's transistors with timing accuracy on the order of hundreds and/or tens of picoseconds, using many signal channels. Accordingly, accurate characterization of signal delay caused by the various tester interface elements, including the tester board, test adapter 125, cable interface, load board 130, and the socket are extremely important. Notably, two values are of particular interest: the absolute delay, i.e., the delay introduced into a single channel, and the relative delay, i.e., the difference between the delays introduced into multiple channels.
Another field of art relating to the invention is the study of semiconductor photoemission. It is known in the art that various mechanisms in semiconductor devices can cause light emission. Detection of such light emission has been used to investigate semiconductor devices. For example, avalanche breakdown in insulators cause light emission, and detection of such light emission can point to the locations of failure in the device. Similar detection can be used to characterize electrostatic discharge in the device. In electrically stimulated (active) transistors, accelerated carriers (electrons & holes), a.k.a. hot-carriers, emit light when the device draws current. Various emission microscopes have been used for detecting locations on the device where the electrical current drawn exceeds the expected levels and therefore could lead to locating failures in semiconductor devices. Some of these hot-carrier emission microscopes have been disclosed in the prior art. See, for example, U.S. Pat. Nos. 4,680,635; 4,811,090; 5,475,316; and 5,940,545; which are incorporated herein in their entirety by reference.
The Emiscope™ is a time-resolved emission (TRE) microscope available from the assignee, Optonics, Inc. (a Credence company), of Mountain View, Calif., and is described in U.S. application Ser. No. 09/995,548, which is incorporated herein in its entirety by reference. The Emiscope™ makes use of the fact that for transistors the photoemission coincides (in-time and characteristics) directly with the voltage transition responsible for the change in the state (logic) of the device. Therefore, it is normally used in conjunction with a tester i.e., the device under test (DUT) is connected to the tester and is being electrically stimulated by it, while the Emiscope™ detects and times emission from the DUT. Using this set-up, one can study the response of the DUT to various electrical stimuli by correlating the Emiscope™ detection signals to the expected signals.
As can be readily understood, timing accuracy of both the tester and the TRE is extremely important. Notably, any time delay introduced by the test adapter, the load board, and other intervening elements must be carefully characterized and accounted for. The present invention provides a method and system for accurate characterization and correction of such delays.